1. Field of the Invention
This invention relates in general to the process and apparatus for opto-electrically scanning and reproducing images.
2. Description of the Prior Art
It has been known to utilize printing cylinders for intaglio printing with existing engraving units. In such apparatus, the image pattern to be reproduced which can contain half-tone pictures and lettering is attached to a rotating scanning cylinder and is scanned, image line by image line, by a light spot scanning member which can be moved axially of the scanning cylinder. The scanning of an image line occurs on a circular circumferential line of the scanning cylinder and after scanning of an image line, the scanning device is moved a distance of two image lines and then the next image line is scanned.
According to the gray tone value of the scanned image dots, more or less light will be reflected from the image pattern onto the scanning device and will be converted by it electro-optically into an analog image signal. The analog image signal is converted to digital information by using a scanning pulse sequence device and the digital data thus obtained is stored into an image line memory. The stored data is read out when desired with the aid of a recording pulse sequence means and reconverted into an analog image signal.
For the production of an intaglio printing raster, a raster signal derived from the recording pulse sequence device is super-imposed on the image signal. The image signal and raster signal control a recording member which may be in the form of a mechanical engraving member having an engraving needle as a cutting tool which is controlled by the information supplied to it.
The raster signal produces a vibrating lifting motion of the engraving needle which, due to this movement, engraves a series of depressions hereinafter referred to as "cuplets" into the rotating printing cylinder. The image signal determines the penetration depth of the engraving needle into the cuplets on the printing cylinder and, ultimately, the gray tone values which will be reproduced.
After a closed circumferential line has been engraved, the engraving tool will be advanced a distance of two image lines and during this time of advancement the engraving is interrupted. After the advancing motion has occurred, the engraving of the next circumferential line begins with a series of cuplets which are arranged such that they are staggered relative to the cuplets of the previously engraved circumferential line.
The scanning cylinder has a fixed circumference and is a component part of the engraving unit, but printing cylinders having differing diameters can be installed on the engraving unit. The scanning cylinder and the printing cylinder are coupled to each other and driven by common synchronous motor. The number of revolutions of the scanning and printing cylinders is adjusted in such a way that when the frequency of the recording pulse sequence device remains constant that an integral number of cuplets is engraved on a closed circumferential line of the printing cylinder which line exactly corresponds to an image line.
The frequency of the scanning pulse sequence is determined such that on an image line from the top to the bottom edge of the image pattern exactly as many image dots are scanned as there are cuplets on the circumferential line of the printing cylinder.
The engraved printing cylinder is the printing form for an intaglio rotary machine. Before the printing operation the cuplets take up more or less ink, according to their volume. During the printing operation, the ink is transferred from the cuplets onto the printing carrier and the gray tone value of a printed surface is determined by the volume of the cuplets arranged in a raster form on this surface and by the amount of ink stored in them. For a multicolor printing operation, a printing cylinder for each color component is engraved. In the intaglio rotary machine during the passage of the printing carrier there occurs the super-imposing of the differently colored printing cylinders of a color set to a color printing. This composite printing must be carried out very exactly so that mismatch, misindexing or misregistrations will not occur. The experts who operate such complicated machinery can detect misregistrations, mismatches and misindexing by watching with the human eye and examining the finished color prints. Misregistrations can occur during the engraving of the printing cylinders because of varying positions of the engraving or of parts of the engraving on the printing cylinders of a color set or, on the other hand, during the printing operation itself by an insufficient and inaccurate synchronism between the printing cylinders and the printing carrier.
Whereas, inaccurate synchronism can be corrected by governing devices, it is not possible to compensate in the printing machine for misregistrations which arise because of inexact positioning of the engraving or of parts of the engraving on the printing cylinder on a color set.
So as to achieve high quality reproduction, the engraving on the printing cylinders must take place with great precision with an exactness of about 1/100 mm. In special engraving processes, the starting and ending of the reproduction must be adjusted with the same degree of accuracy.
The special engraving processes known to the experts as frame suppression, subsequent engraving and seamless engraving will be briefly described.
Frequently, the image pattern to be reproduced has margins running in the axial direction of the scanning cylinder which contain, for example, the perforations of an accurate fitting arrangement of the image pattern on the scanning cylinder with the aid of register pegs. It can also happen that the image pattern does not completely fill the cylinder's circumference and a part of the cylinder surface is visible.
Since the scanning member scans the entire circumferential surface of the scanning cylinder, however, the beginning and end of the scanning must be determined in such a way that the image signal only reaches the engraving member when the scanning member is scanning the information carrying part of the image pattern. On the other hand, information picked up by the scanning member which is not to be recorded can also be eliminated by suppression of the image signal with the use of controlling signals.
On prior art engraving units, these control signals are obtained from pulse generators consisting of a drum attached to the cylinder axis and utilizing a fixed scanner.
On the drum, there are reflectors arranged which at a predetermined momentary position of the cylinder relative to the scanning member reflect into the fixed scanner the light emanating from a light source. The scanner generates by opto-electrical conversion means, electronic control signals from the reflected light which can be used to control the image signal for the purpose of frame suppression.
In practice it is very difficult to position the reflectors with the required matching preciseness so that the control signals appear at precisely that point in time at which the top or bottom edges of the image pattern move beneath the scanning member. The adjustment of the beginning and end of scanning can on the prior art engraving units not be accomplished under operating conditions during the rotation of the cylinders but must be accomplished while the cylinders are stationary.